The air-lift reactor is a mechanically simple, combined gas-liquid flow device characterized by fluid circulation in a defined cyclic pattern through specifically designed channels. Fluid motion is due to the mean density difference in the upflow section, i.e., the riser and the downflow section, i.e., the downcomer. The air-lift reactor is comprised of distinct zones with different flow patterns. The riser is the zone where the gas is injected creating a fluid density difference. This zone exhibits cocurrent upward flow of both liquid and gas phases. At the top of the reactor is the gas-liquid separator section. This is a region of horizontal fluid flow and flow reversal where gas bubbles disengage from the liquid phase. The downcomer is the zone where the gas-liquid dispersion or degassed liquid recirculates to the riser. The downcomer zone exhibits either single-phase, two-phase cocurrent, or two-phase mixed cocurrent-countercurrent downward flow, depending on whether the liquid velocity is greater than the free-rise velocity of the bubbles. The base section at the lower end of the vessel communicates the exit of the downcomer to the entrance of the riser.
The air-lift reactor has been predominately utilized for microorganism fermentation processes and single-cell protein production. Little work has been done to develop a wastewater treatment process for the application of air-lift reactor technology. Examples of air-lift reactors developed for use as wastewater treatment processes include the Betz reactor (Gasner, L. L., 1974, Development and application of the thin channel rectangular air lift mass transfer reactor to fermentation and waste-water treatment systems. Biotech. Bioeng., 16:1179-1195), and the I.C.I. "Deep Shaft" reactors (Hines et al., 1975, The ICI deep shaft aeration process for effluent treatment. Inst. Chem. Eng. Sym. Ser. U.K., 41:D1-D10). However, continuous flow air-lift reactors generally are disadvantaged in the area of settling and bulking control, denitrification and phosphate removal without chemical addition and good oxygen transfer efficiency relative to other treatment mechanisms.